EARTH

					                                             EARTH

Earth (or the Earth) is the third planet from the Sun, and the densest and fifth-largest of the
eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial
planets. It is sometimes referred to as the World, the Blue Planet,[20] or by its Latin name,
Terra.[note 6]

Earth formed 4.54 billion years ago, and life appeared on its surface within one billion
years.[21] The planet is home to millions of species, including humans.[22] Earth's biosphere
has significantly altered the atmosphere and other abiotic conditions on the planet, enabling
the proliferation of aerobic organisms as well as the formation of the ozone layer which,
together with Earth's magnetic field, blocks harmful solar radiation, permitting life on
land.[23] The physical properties of the Earth, as well as its geological history and orbit, have
allowed life to persist during this period. The planet is expected to continue supporting life
for at least another 500 million years.[24][25]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that migrate
across the surface over periods of many millions of years. About 71% of the surface is
covered by salt water oceans, with the remainder consisting of continents and islands which
together have many lakes and other sources of water that contribute to the hydrosphere.
Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice
cap). The planet's interior remains active, with a thick layer of relatively solid mantle, a liquid
outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth
orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26
solar days, or one sidereal year.[note 7] The Earth's axis of rotation is tilted 23.4° away from
the perpendicular of its orbital plane, producing seasonal variations on the planet's surface
with a period of one tropical year (365.24 solar days).[26] Earth's only known natural
satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides,
stabilizes the axial tilt, and gradually slows the planet's rotation. Between approximately 3.8
billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy
Bombardment caused significant changes to the greater surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute
resources that are used to support a global human population. These inhabitants are grouped
into about 200 independent sovereign states, which interact through diplomacy, travel, trade,
and military action. Human cultures have developed many views of the planet, including
personification as a deity, a belief in a flat Earth or in the Earth as the center of the universe,
and a modern perspective of the world as an integrated environment that requires
stewardship.

Chronology

Main article: History of the Earth

See also: Geological history of Earth
The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago,[27]
and by 4.54 billion years ago (within an uncertainty of 1%)[21] the Earth and the other
planets in the Solar System had formed out of the solar nebula—a disk-shaped mass of dust
and gas left over from the formation of the Sun. This assembly of the Earth through accretion
was thus largely completed within 10–20 million years.[28] Initially molten, the outer layer
of the planet Earth cooled to form a solid crust when water began accumulating in the
atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago.[29]

The current consensus model[30] for the formation of the Moon is the giant impact
hypothesis, in which the Moon was created when a Mars-sized object (sometimes called
Theia) with about 10% of the Earth's mass[31] impacted the Earth in a glancing blow.[32] In
this model, some of this object's mass would have merged with the Earth and a portion would
have been ejected into space, but enough material would have been sent into orbit to coalesce
into the Moon.

Outgassing and volcanic activity produced the primordial atmosphere of the Earth.
Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the
larger proto-planets, comets, and trans-Neptunian objects produced the oceans.[33] The
newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early
oceans remained liquid—a contradiction dubbed the faint young Sun paradox. A combination
of greenhouse gases and higher levels of solar activity served to raise the Earth's surface
temperature, preventing the oceans from freezing over.[34] By 3.5 billion years ago, the
Earth's magnetic field was established, which helped prevent the atmosphere from being
stripped away by the solar wind.[35]

Two major models have been proposed for the rate of continental growth:[36] steady growth
to the present-day[37] and rapid growth early in Earth history.[38] Current research shows
that the second option is most likely, with rapid initial growth of continental crust[39]
followed by a long-term steady continental area.[40][41][42] On time scales lasting hundreds
of millions of years, the surface continually reshaped as continents formed and broke up. The
continents migrated across the surface, occasionally combining to form a supercontinent.
Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia,
began to break apart. The continents later recombined to form Pannotia, 600–540 Ma, then
finally Pangaea, which broke apart 180 Ma.[43]

Evolution of life

Main article: Evolutionary history of life

Highly energetic chemistry is believed to have produced a self-replicating molecule around 4
billion years ago and half a billion years later the last common ancestor of all life existed.[44]
The development of photosynthesis allowed the Sun's energy to be harvested directly by life
forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a
form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells
within larger ones resulted in the development of complex cells called eukaryotes.[45] True
multicellular organisms formed as cells within colonies became increasingly specialized.
Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the
surface of Earth.[46]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma,
during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has
been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian
explosion, when multicellular life forms began to proliferate.[47]

Following the Cambrian explosion, about 535 Ma, there have been five major mass
extinctions.[48] The most recent such event was 65 Ma, when an asteroid impact triggered
the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small
animals such as mammals, which then resembled shrews. Over the past 65 million years,
mammalian life has diversified, and several million years ago an African ape-like animal
such as Orrorin tugenensis gained the ability to stand upright.[49] This enabled tool use and
encouraged communication that provided the nutrition and stimulation needed for a larger
brain, which allowed the evolution of the human race. The development of agriculture, and
then civilization, allowed humans to influence the Earth in a short time span as no other life
form had,[50] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene
about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and
thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years
ago.[51]

Future

Main article: Future of the Earth

See also: Risks to civilization, humans and planet Earth




The life cycle of the Sun

The future of the planet is closely tied to that of the Sun. As a result of the steady
accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The
luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40%
over the next 3.5 Gyr.[52] Climate models indicate that the rise in radiation reaching the
Earth is likely to have dire consequences, including the loss of the planet's oceans.[53]

The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing
its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in
approximately 500 million[24] to 900 million years. The lack of vegetation will result in the
loss of oxygen in the atmosphere, so animal life will become extinct within several million
more years.[54] After another billion years all surface water will have disappeared[25] and
the mean global temperature will reach 70 °C[54] (158 °F). The Earth is expected to be
effectively habitable for about another 500 million years from that point,[24] although this
may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.[55]
Even if the Sun were eternal and stable, the continued internal cooling of the Earth would
result in a loss of much of its CO2 due to reduced volcanism,[56] and 35% of the water in the
oceans would descend to the mantle due to reduced steam venting from mid-ocean
ridges.[57]

The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that
the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000
km).[52][58] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its
mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from
the Sun when the star reaches it maximum radius. The planet was therefore initially expected
to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not
all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at
about 5000 times its present level).[52] A 2008 simulation indicates that Earth's orbit will
decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be
vaporized.[58]

Composition and structure

Main article: Earth science

Further information: Earth physical characteristics tables

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like
Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four
planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic
field, and fastest rotation,[59] and is probably the only one with active plate tectonics.[60]

Shape

Main article: Figure of the Earth




Size comparison of inner planets (left to right): Mercury, Venus, Earth and Mars

The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis
from pole to pole such that there is a bulge around the equator.[61] This bulge results from
the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the
pole to pole diameter.[62] The average diameter of the reference spheroid is about 12,742
km, which is approximately 40,000 km/π, as the meter was originally defined as
1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[63]

Local topography deviates from this idealized spheroid, although on a global scale, these
deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the
reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[64] The
largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above
local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the
equatorial bulge, the surface locations farthest from the center of the Earth are the summits of
Mount Chimborazo in Ecuador and Huascarán in Peru.[65][66][67]

Chemical composition of the crust[68]


Compound          Formula       Composition

                  Continental   Oceanic

silica

SiO2 60.2% 48.6%

alumina

Al2O3 15.2% 16.5%

lime

CaO      5.5% 12.3%

magnesia

MgO 3.1% 6.8%

iron(II) oxide

FeO      3.8% 6.2%

sodium oxide

Na2O 3.0% 2.6%

potassium oxide

K2O      2.8% 0.4%

iron(III) oxide

Fe2O3 2.5% 2.3%

water

H2O      1.4% 1.1%

carbon dioxide

CO2      1.2% 1.4%
titanium dioxide

TiO2 0.7% 1.4%

phosphorus pentoxide

P2O5 0.2% 0.3%

Total 99.6% 99.9%

Chemical composition

See also: Abundance of elements on Earth

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%),
oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium
(1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other
elements. Due to mass segregation, the core region is believed to be primarily composed of
iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace
elements.[69]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust
consists of oxygen. The more common rock constituents of the Earth's crust are nearly all
oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total
amount in any rock is usually much less than 1%. The principal oxides are silica, alumina,
iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid,
forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a
computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were
composed of 11 oxides (see the table at right), with the other constituents occurring in minute
quantities.[70]

Internal structure

Main article: Structure of the Earth

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by
their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it
has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate
solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from
the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging
6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of
the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the
tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-
viscosity layer on which the lithosphere rides. Important changes in crystal structure within
the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that
separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid
outer core lies above a solid inner core.[71] The inner core may rotate at a slightly higher
angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[72]
Geologic layers of the Earth[73]




Earth cutaway from core to exosphere. Not to scale. Depth[74]

km      Component Layer        Density

g/cm3

        0–60   Lithosphere[note 8]

—

        0–35   Crust[note 9]

2.2–2.9

        35–60 Upper mantle 3.4–4.4

          35–2890      Mantle 3.4–5.6

        100–700        Asthenosphere —

        2890–5100      Outer core    9.9–12.2

        5100–6378      Inner core    12.8–13.1

Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion
(about 20%) and heat produced through radioactive decay (80%).[75] The major heat-
producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-
232.[76] At the center of the planet, the temperature may be up to 7,000 K and the pressure
could reach 360 GPa.[77] Because much of the heat is provided by radioactive decay,
scientists believe that early in Earth history, before isotopes with short half-lives had been
depleted, Earth's heat production would have been much higher. This extra heat production,
twice present-day at approximately 3 billion years ago,[75] would have increased temperature
gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and
allowing the production of igneous rocks such as komatiites that are not formed today.[78]

Present-day major heat-producing isotopes[79]


Isotope Heat release

W/kg isotope
Half-life



years Mean mantle concentration

kg isotope/kg mantle Heat release

W/kg mantle

238U 9.46 × 10−5         4.47 × 109    30.8 × 10−9   2.91 × 10−12

235U 5.69 × 10−4         7.04 × 108    0.22 × 10−9   1.25 × 10−13

232Th 2.64 × 10−5        1.40 × 1010   124 × 10−9    3.27 × 10−12

40K    2.92 × 10−5       1.25 × 109    36.9 × 10−9   1.08 × 10−12

The mean heat loss from the Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013
W.[80] A portion of the core's thermal energy is transported toward the crust by mantle
plumes; a form of convection consisting of upwellings of higher-temperature rock. These
plumes can produce hotspots and flood basalts.[81] More of the heat in the Earth is lost
through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final
major mode of heat loss is through conduction through the lithosphere, the majority of which
occurs in the oceans because the crust there is much thinner than that of the continents.[82]

Tectonic plates

Earth's main plates[83]




Plate name        Area

106 km2

African Plate[note 10]

78.0

Antarctic Plate

60.9

Indo-Australian Plate

47.2

Eurasian Plate

67.8
North American Plate

75.9

South American Plate

43.6

Pacific Plate

103.3

Main article: Plate tectonics

The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called
tectonic plates. These plates are rigid segments that move in relation to one another at one of
three types of plate boundaries: Convergent boundaries, at which two plates come together,
Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in
which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-
building, and oceanic trench formation can occur along these plate boundaries.[84] The
tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper
mantle that can flow and move along with the plates,[85] and their motion is strongly coupled
with convection patterns inside the Earth's mantle.

As the tectonic plates migrate across the planet, the ocean floor is subducted under the
leading edges of the plates at convergent boundaries. At the same time, the upwelling of
mantle material at divergent boundaries creates mid-ocean ridges. The combination of these
processes continually recycles the oceanic crust back into the mantle. Because of this
recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic
crust is located in the Western Pacific, and has an estimated age of about 200 million
years.[86][87] By comparison, the oldest dated continental crust is 4030 million years
old.[88]

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-
Australian, and South American. Other notable plates include the Arabian Plate, the
Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in
the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and
55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate
advancing at a rate of 75 mm/yr[89] and the Pacific Plate moving 52–69 mm/yr. At the other
extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about
21 mm/yr.[90]

Surface

Main articles: Landform and Extreme points of Earth

The Earth's terrain varies greatly from place to place. About 70.8%[91] of the surface is
covered by water, with much of the continental shelf below sea level. The submerged surface
has mountainous features, including a globe-spanning mid-ocean ridge system, as well as
undersea volcanoes,[62] oceanic trenches, submarine canyons, oceanic plateaus and abyssal
plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains,
plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods because of tectonics
and erosion. The surface features built up or deformed through plate tectonics are subject to
steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation,
coastal erosion, the build-up of coral reefs, and large meteorite impacts[92] also act to
reshape the landscape.




Present day Earth altimetry and bathymetry. Data from the National Geophysical Data
Center's TerrainBase Digital Terrain Model.

The continental crust consists of lower density material such as the igneous rocks granite and
andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the
ocean floors.[93] Sedimentary rock is formed from the accumulation of sediment that
becomes compacted together. Nearly 75% of the continental surfaces are covered by
sedimentary rocks, although they form only about 5% of the crust.[94] The third form of rock
material found on Earth is metamorphic rock, which is created from the transformation of
pre-existing rock types through high pressures, high temperatures, or both. The most
abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole,
mica, pyroxene and olivine.[95] Common carbonate minerals include calcite (found in
limestone) and dolomite.[96]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil
formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and
biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71%
supporting permanent crops.[13] Close to 40% of the Earth's land surface is presently used
for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of
pastureland.[97]

The elevation of the land surface of the Earth varies from the low point of −418 m at the
Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest.
The mean height of land above sea level is 840 m.[98]

Hydrosphere

Main article: Hydrosphere
Elevation histogram of the surface of the Earth

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue
Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the
oceans, but technically includes all water surfaces in the world, including inland seas, lakes,
rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location
is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4
m.[note 11][99]

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total
mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682
m, resulting in an estimated volume of 1.332×109 km3.[100] If all the land on Earth were
spread evenly, water would rise to an altitude of more than 2.7 km.[note 12] About 97.5% of
the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%,
is currently ice.[101]

The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water
(35 ‰).[102] Most of this salt was released from volcanic activity or extracted from cool,
igneous rocks.[103] The oceans are also a reservoir of dissolved atmospheric gases, which
are essential for the survival of many aquatic life forms.[104] Sea water has an important
influence on the world's climate, with the oceans acting as a large heat reservoir.[105] Shifts
in the oceanic temperature distribution can cause significant weather shifts, such as the El
Niño-Southern Oscillation.[106]

Atmosphere

Main article: Atmosphere of Earth

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale
height of about 8.5 km.[3] It is 78% nitrogen and 21% oxygen, with trace amounts of water
vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with
latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation
resulting from weather and seasonal factors.[107]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved
2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere of today. This
change enabled the proliferation of aerobic organisms as well as the formation of the ozone
layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric
functions important to life on Earth include transporting water vapor, providing useful gases,
causing small meteors to burn up before they strike the surface, and moderating
temperature.[108] This last phenomenon is known as the greenhouse effect: trace molecules
within the atmosphere serve to capture thermal energy emitted from the ground, thereby
raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the
primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the
average surface temperature would be −18 °C and life would likely not exist.[91]

Weather and climate
Main articles: Weather and Climate




Satellite cloud cover image of Earth using NASA's Moderate-Resolution Imaging
Spectroradiometer

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into
outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of
the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats
this layer, and the surface below, causing expansion of the air. This lower density air then
rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that
drives the weather and climate through redistribution of heat energy.[109]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region
below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[110] Ocean
currents are also important factors in determining climate, particularly the thermohaline
circulation that distributes heat energy from the equatorial oceans to the polar regions.[111]

Water vapor generated through surface evaporation is transported by circulatory patterns in
the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water
condenses and settles to the surface as precipitation.[109] Most of the water is then
transported to lower elevations by river systems and usually returned to the oceans or
deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is
a primary factor in the erosion of surface features over geological periods. Precipitation
patterns vary widely, ranging from several meters of water per year to less than a millimeter.
Atmospheric circulation, topological features and temperature differences determine the
average precipitation that falls in each region.[112]

The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher
latitudes the sunlight reaches the surface at a lower angles and it must pass through thicker
columns of the atmosphere. As a result, the mean annual air temperature at sea level
decreases by about 0.4°C per per degree of latitude away from the equator.[113] The Earth
can be sub-divided into specific latitudinal belts of approximately homogeneous climate.
Ranging from the equator to the polar regions, these are the tropical (or equatorial),
subtropical, temperate and polar climates.[114] Climate can also be classified based on the
temperature and precipitation, with the climate regions characterized by fairly uniform air
masses. The commonly used Köppen climate classification system (as modified by Wladimir
Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle
latitudes, continental and cold polar), which are further divided into more specific
subtypes.[110]

Upper atmosphere
This view from orbit shows the full Moon partially obscured and deformed by the Earth's
atmosphere. NASA image

See also: Outer space

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere,
and thermosphere.[108] Each layer has a different lapse rate, defining the rate of change in
temperature with height. Beyond these, the exosphere thins out into the magnetosphere,
where the Earth's magnetic fields interact with the solar wind.[115] Within the stratosphere is
the ozone layer, a component that partially shields the surface from ultraviolet light and thus
is important for life on Earth. The Kármán line, defined as 100 km above the Earth's surface,
is a working definition for the boundary between atmosphere and space.[116]

Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere
have their velocity increased to the point where they can escape from the planet's gravity.
This results in a slow but steady leakage of the atmosphere into space. Because unfixed
hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks
into outer space at a greater rate than other gasses.[117] The leakage of hydrogen into space
contributes to the pushing of the Earth from an initially reducing state to its current oxidizing
one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as
hydrogen is believed to have been a necessary precondition for the widespread accumulation
of oxygen in the atmosphere.[118] Hence the ability of hydrogen to escape from the Earth's
atmosphere may have influenced the nature of life that developed on the planet.[119] In the
current, oxygen-rich atmosphere most hydrogen is converted into water before it has an
opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of
methane in the upper atmosphere.[120]

Magnetic field




Schematic of Earth's magnetosphere. The solar wind flows from left to right

Main article: Earth's magnetic field

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently
located proximate to the planet's geographic poles. At the equator of the magnetic field, the
magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole
moment of 7.91 × 1015 T m3.[121] According to dynamo theory, the field is generated
within the molten outer core region where heat creates convection motions of conducting
materials, generating electric currents. These in turn produce the Earth's magnetic field. The
convection movements in the core are chaotic; the magnetic poles drift and periodically
change alignment. This results in field reversals at irregular intervals averaging a few times
every million years. The most recent reversal occurred approximately 700,000 years
ago.[122][123]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward
edge of the bow shock is located at about 13 times the radius of the Earth. The collision
between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of
concentric, torus-shaped regions of energetic charged particles. When the plasma enters the
Earth's atmosphere at the magnetic poles, it forms the aurora.[124]

Orbit and rotation

Rotation

Main article: Earth's rotation




Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean
solar time (86,400.0025 SI seconds).[125] As the Earth's solar day is now slightly longer than
it was during the 19th century because of tidal acceleration, each day varies between 0 and 2
SI ms longer.[126][127]

Earth's rotation period relative to the fixed stars, called its stellar day by the International
Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean
solar time (UT1), or 23h 56m 4.098903691s.[2][note 13] Earth's rotation period relative to
the precessing or moving mean vernal equinox, misnamed its sidereal day, is
86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).[2] Thus
the sidereal day is shorter than the stellar day by about 8.4 ms.[128] The length of the mean
solar day in SI seconds is available from the IERS for the periods 1623–2005[129] and 1962–
2005.[130]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent
motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. For
bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or
Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the
Moon are approximately the same.[131][132]

Orbit

Main article: Earth's orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564
mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the
Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter,
every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth
to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital
speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to cover
the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon
(384,000 km) in four hours.[3]

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to
the background stars. When combined with the Earth–Moon system's common revolution
around the Sun, the period of the synodic month, from new moon to new moon, is 29.53
days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial
rotations are all counter-clockwise. Viewed from a vantage point above the north poles of
both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about
the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4
degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted
about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every
two weeks, alternating between lunar eclipses and solar eclipses.[3][133]

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or
1,500,000 kilometers) in radius.[134][note 14] This is maximum distance at which the Earth's
gravitational influence is stronger than the more distant Sun and planets. Objects must orbit
the Earth within this radius, or they can become unbound by the gravitational perturbation of
the Sun.




Illustration of the Milky Way Galaxy, showing the location of the Sun

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about
28,000 light years from the center of the galaxy. It is currently about 20 light years above the
galaxy's equatorial plane in the Orion spiral arm.[135]

Axial tilt and seasons

Main article: Axial tilt

Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the
surface varies over the course of the year. This results in seasonal change in climate, with
summer in the northern hemisphere occurring when the North Pole is pointing toward the
Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts
longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler
and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no
daylight at all for part of the year—a polar night. In the southern hemisphere the situation is
exactly reversed, with the South Pole oriented opposite the direction of the North Pole.
Earth and Moon from Mars, imaged by Mars Reconnaissance Orbiter. From space, the Earth
can be seen to go through phases similar to the phases of the Moon.

By astronomical convention, the four seasons are determined by the solstices—the point in
the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the
direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere,
Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring
Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern
hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and
the Spring and Autumnal Equinox dates switched.[136]

The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does
undergo nutation; a slight, irregular motion with a main period of 18.6 years.[137] The
orientation (rather than the angle) of the Earth's axis also changes over time, precessing
around in a complete circle over each 25,800 year cycle; this precession is the reason for the
difference between a sidereal year and a tropical year. Both of these motions are caused by
the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the
perspective of the Earth, the poles also migrate a few meters across the surface. This polar
motion has multiple, cyclical components, which collectively are termed quasiperiodic
motion. In addition to an annual component to this motion, there is a 14-month cycle called
the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known
as length of day variation.[138]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4.
These dates change over time due to precession and other orbital factors, which follow
cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in
an increase of about 6.9%[note 15] in solar energy reaching the Earth at perihelion relative to
aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that
the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly
more energy from the Sun than does the northern over the course of a year. This effect is
much less significant than the total energy change due to the axial tilt, and most of the excess
energy is absorbed by the higher proportion of water in the southern hemisphere.[139]

Moon

Characteristics

Diameter          3,474.8 km

Mass 7.349×1022 kg

Semi-major axis

384,400 km

Orbital period 27 d 7 h 43.7 m
Main article: Moon

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-
quarter of the Earth's. It is the largest moon in the Solar System relative to the size of its
planet, although Charon is larger relative to the dwarf planet Pluto. The natural satellites
orbiting other planets are called "moons" after Earth's Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. The same
effect on the Moon has led to its tidal locking: its rotation period is the same as the time it
takes to orbit the Earth. As a result, it always presents the same face to the planet. As the
Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar
phases; the dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately
38 mm a year. Over millions of years, these tiny modifications—and the lengthening of
Earth's day by about 23 µs a year—add up to significant changes.[140] During the Devonian
period, for example, (approximately 410 million years ago) there were 400 days in a year,
with each day lasting 21.8 hours.[141]




Details of the Earth-Moon system. Besides the radius of each object, the radius to the Earth-
Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is
located by Cassini's third law.

The Moon may have dramatically affected the development of life by moderating the planet's
climate. Paleontological evidence and computer simulations show that Earth's axial tilt is
stabilized by tidal interactions with the Moon.[142] Some theorists believe that without this
stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge,
the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of
years, as appears to be the case for Mars.[143]

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-
sized disk as the Sun. The angular size (or solid angle) of these two bodies match because,
although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times
more distant.[132] This allows total and annular solar eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it
formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This
hypothesis explains (among other things) the Moon's relative lack of iron and volatile
elements, and the fact that its composition is nearly identical to that of the Earth's crust.[144]

Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002
AA29.[145][146] As of 2011, there are 931 operational, man-made satellites orbiting the
Earth.[147] On July 27, 2011, astronomers reported a trojan asteroid companion, 2010 TK7,
librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the
Sun.[148][149]




A scale representation of the relative sizes of, and average distance between, Earth and Moon

Habitability

See also: Planetary habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The
Earth provides liquid water—an environment where complex organic molecules can
assemble and interact, and sufficient energy to sustain metabolism.[150] The distance of the
Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological
history, sustaining atmosphere and protective magnetic field all contribute to the current
climactic conditions at the surface.[151]

Biosphere

Main article: Biosphere

The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally
believed to have begun evolving about 3.5 billion years ago. The biosphere is divided into a
number of biomes, inhabited by broadly similar plants and animals. On land, biomes are
separated primarily by differences in latitude, height above sea level and humidity. Terrestrial
biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid
areas are relatively barren of plant and animal life; species diversity reaches a peak in humid
lowlands at equatorial latitudes.[152]

Natural resources and land use

Main article: Natural resource

The Earth provides resources that are exploitable by humans for useful purposes. Some of
these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a
short time scale.

Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal,
petroleum, natural gas and methane clathrate. These deposits are used by humans both for
energy production and as feedstock for chemical production. Mineral ore bodies have also
been formed in Earth's crust through a process of Ore genesis, resulting from actions of
erosion and plate tectonics.[153] These bodies form concentrated sources for many metals
and other useful elements.
The Earth's biosphere produces many useful biological products for humans, including (but
far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic
wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic
ecosystem depends upon dissolved nutrients washed down from the land.[154] Humans also
live on the land by using building materials to construct shelters. In 1993, human use of land
is approximately:

Land use       Arable land    Permanent crops        Permanent pastures     Forests and
woodland       Urban areas    Other

Percentage     13.13%[13]

4.71%[13]

26%    32%     1.5% 30%

The estimated amount of irrigated land in 1993 was 2,481,250 km2.[13]

Natural and environmental hazards

Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones,
hurricanes, or typhoons that dominate life in those areas. From 1980–2000, these events
caused an average of 11,800 deaths per year.[155] Many places are subject to earthquakes,
landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts,
wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and
toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of
wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of
invasive species.

According to the United Nations, a scientific consensus exists linking human activities to
global warming due to industrial carbon dioxide emissions. This is predicted to produce
changes such as the melting of glaciers and ice sheets, more extreme temperature ranges,
significant changes in weather and a global rise in average sea levels.[156]

Human geography

Main article: Human geography

See also: World




7 continents of Earth:[157] North America ,       South America,      Antarctica,    Africa,
Europe,    Asia, Australia
Cartography, the study and practice of map making, and vicariously geography, have
historically been the disciplines devoted to depicting the Earth. Surveying, the determination
of locations and distances, and to a lesser extent navigation, the determination of position and
direction, have developed alongside cartography and geography, providing and suitably
quantifying the requisite information.

Earth has approximately 6,910,000,000 human inhabitants as of April 25, 2011.[158]
Projections indicate that the world's human population will reach 7 billion in early 2012 and
9.2 billion in 2050.[159] Most of the growth is expected to take place in developing nations.
Human population density varies widely around the world, but a majority live in Asia. By
2020, 60% of the world's population is expected to be living in urban, rather than rural,
areas.[160]

It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live
on—three-quarters is covered by oceans, and half of the land area is either desert (14%),[161]
high mountains (27%),[162] or other less suitable terrain. The northernmost permanent
settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[163] (82°28′N) The
southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the
South Pole. (90°S)




The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated
night-time image of the world. This image is not photographic and many features are brighter
than they would appear to a direct observer.

Independent sovereign nations claim the planet's entire land surface, except for some parts of
Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2011
there are 204 sovereign states, including the 193 United Nations member states. In addition,
there are 59 dependent territories, and a number of autonomous areas, territories under
dispute and other entities.[13] Historically, Earth has never had a sovereign government with
authority over the entire globe, although a number of nation-states have striven for world
domination and failed.[164]

The United Nations is a worldwide intergovernmental organization that was created with the
goal of intervening in the disputes between nations, thereby avoiding armed conflict.[165]
The U.N. serves primarily as a forum for international diplomacy and international law.
When the consensus of the membership permits, it provides a mechanism for armed
intervention.[166]

The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[167] In total, about
400 people visited outer space and reached Earth orbit as of 2004, and, of these, twelve have
walked on the Moon.[168][169][170] Normally the only humans in space are those on the
International Space Station. The station's crew, currently six people, is usually replaced every
six months.[171] The furthest humans have travelled from Earth is 400,171 km, achieved
during the 1970 Apollo 13 mission

				
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